Summary of Work: An obligate step in the life cycle of HIV is the integration of the viral DNA into the host chromosome. This reaction is mediated by the integrase enzyme (In) through the formation of a staggered double-strand break (DSB) in the host DNA and covalent linkage of the viral DNA ends to the host DNA. We have developed yeast based genetic detection systems to investigate in-vivo mechanisms of In, as well as means of altering the reaction, since yeast has repair and recombination systems that can identify and process DNA lesions including DNA breaks. We have examined the effects of overexpression of HIV In. Using two different isolates of HIV In overexpressed at various levels in wildtype and various mutant yeast strains there was no change in growth, morphology, viability of the yeast mutants, or increased sensitivity to various DNA damaging agents. Included were rad9 and rad17 mutants defective in cell cycle checkpoint control, rad50 and xrs2 mutants defective in Ku mediated end-joining, and rad52 mutants defective in double-strand break repair. A variety of yeast single and double mutants affecting many aspects of recombination and DSB and single-strand break repair and which have been characterized recently in our studies of in vivo synthesized endonucleases are currently being examined for their responses to induced HIV Integrase.